1. Field of the Invention
The present invention relates to a technique for designing the layout of a semiconductor integrated circuit and, more particularly, to a semiconductor integrated circuit device designed by arranging standard cells.
2. Description of the Related Art
Conventionally, the layout of a semiconductor integrated circuit is designed by arranging circuit components called standard cells. For example, standard cells are arranged in a horizontal direction to form a standard cell row, and a plurality of standard cell rows are arranged in a vertical direction to form a circuit block. Each standard cell has a PMOS transistor and an NMOS transistor. In the N-well structure, the PMOS transistor is formed on an N well and the NMOS transistor is formed on a P substrate. In the twin-well structure, the PMOS transistor is formed on an N well and the NMOS transistor is formed on a P well.
Also, in order to achieve low power consumption required by recent mobile apparatuses, a standard cell block is formed on a triple well so that a power supply control technique or a substrate control technique is applied. In the triple-well structure, the PMOS transistor is formed on an N well and the NMOS transistor is formed on a P well, and the N well and the P well are formed on a triple-well that is a deep N well. In addition, the P well is separated from a P substrate by the triple-well. FIG. 48 schematically shows the triple-well structure.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-133416
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2007-165670
With the development of even smaller microstructures, a phenomenon called a well proximity effect is more likely to occur. The well proximity effect is a phenomenon such that, when impurities are implanted into a well, the impurities are reflected and scattered from a resist and are then implanted into a channel region of a transistor, so that the impurity concentration of the channel exceeds a set value, resulting in an increase in threshold value of the transistor.
The impurity implantation amount due to the reflection and scattering differs, depending on a space between the transistor and the well, and tends to increase with a decrease in the space. The space between the transistor and the well as used herein corresponds to a distance from the transistor to an end of the well. In smaller microstructures, the space between the transistor and the well (layout rule) is further narrowed, so that the well proximity effect is more likely to occur as a side effect.
Typically, in the step of logic design of an electronic circuit, it is assumed that the same cells have the same characteristics. However, when the space between a transistor and a well differs on the layout, the transistor characteristics of a product may differ due to the influence of the well proximity effect. Therefore, there is a mismatch between the design and the product in terms of circuit operation timing or the like, resulting in a defective product. In addition, a decrease in yield, a decrease in circuit performance due to addition of a design margin for securing a difference in characteristics, and an increase in block area lead to a reduction in competitiveness.
On the other hand, in order to cause the same cells to have the same characteristics, the space between the transistor and the well needs to be constant or be broadened to an extent that can prevent reflected and scattered impurities from reaching so the influence of the well proximity effect is negligible.
An exemplary portion where variations in characteristics due to the well proximity effect occur is an end portion of a standard cell row. Specifically, the space between a transistor and a well is considerably large in the vicinity of a center of a cell row since a cell is interposed between adjacent cells on both sides thereof, whereas there is only a space having a well width included in a single cell at an end portion of a cell row. Therefore, a difference occurs in characteristics.
Also, a difference in characteristics is likely to occur at the uppermost or lowermost standard cell row. In a typical semiconductor integrated circuit designed using standard cells, every other standard cell row is flipped, and two vertically adjacent standard cell rows share a well region. In other words, the width of a well region is broad with the exception of the uppermost row or the lowermost row. Therefore, the uppermost or lowermost standard cell row has a smaller well width than those of the other portions, so that a difference in characteristics occurs therein.
The above-described problem has become significant with the development of even smaller microstructures. Conventionally, no measures in which the influence of the well proximity effect and the space between a transistor and a well are taken into consideration have not been taken in layout design.